Conventionally, there is provided a lighting system which irradiates a light in a desired color temperature by mixing different kinds of lights having different wavelengths (see, e.g., Japanese Patent Application Publication No. 2009-518799 (JP2009-518799A), paragraphs [0021] to [0023] and FIG. 1). The lighting system includes an illumination unit having a plurality of LEDs which emit lights of different wavelengths and an adjustment unit which adjusts the color temperature of a light emitted from the illumination unit. In the lighting system, when a color adjusting operation is conducted by a user, the adjustment unit adjusts a light output of each LED so that the light emitted from the illumination unit has a color temperature corresponding to the color adjusting operation.
As in the lighting system described in JP2009-518799A, when the output of LED of each color is adjusted based on a single-level signal according to the color mixing operation, the LEDs of the respective colors may not produce lights corresponding to a set output ratio in an excessive change in the output. The cause of this is explained as follows.
For example, in order to continually change a current flowing through the LEDs of respective colors and the color temperature of the light emitted from the illumination unit according to the output controlled by a microcomputer, a PWM (Pulse Width Modulation) timer circuit is preferably used to suppress an increase in cost. In this case, an output of the PWM timer circuit provided for the LED of each color is smoothed by a low pass filter to be used as an instruction value of the current flowing through the LED of each color. Further, the frequency of the PWM timer circuit is set to a value ranging from several hundred Hz to several kHz, and a time constant of the low pass filter is set to sufficiently smooth the frequency of the PWM timer circuit. For example, when the frequency of the PWM timer circuit is 1 kHz, the low pass filter may have a time constant of 0.16, which is sufficient to smooth the frequency of the PWM timer circuit.
Here, in a stable operation, the ratio between currents flowing through the LEDs of respective colors is determined based on a value stored in a memory of a microcomputer. In an excessive change of the output, however, the ratio may deviate from a set value due to the delay of the low pass filter smoothing the output of the PWM timer circuit. In detail, the currents flowing through the LEDs of respective colors continuously change from current values before a change of setting to current values after the change of setting based on a property of the low pass filter during the response time of the low pass filter and the currents flowing through the LEDs of the respective colors are set regardless of previously determined values. Thus, a light of an unexpected color is momentarily emitted. Such a problem is overcome by reducing the delay of the low pass filter smoothing the output of the PWM timer circuit. However, decreasing the delay of the low pass filter changes the currents flowing through the LEDs of respective colors in a period of the PWM timer circuit, thus causing flicker.
In order to prevent the flicker, the frequency of the PWM timer circuit may be increased. However, it is impossible to considerably increase the frequency of the PWM timer circuit due to a limit to a resolution of a current set value. For example, in a timer clock of 20 MHz, a 16-bit timer has a resolution of 50 ns. When an output of the PWM timer circuit is continuously changed with the resolution of the 16 bit-timer, one period is 65536 counts, and the frequency becomes about 300 Hz. Further, in a case of the resolution of a 14 bit-timer, the frequency is about 1.2 kHz. In order to further increase the frequency, the resolution is reduced, i.e., the timer clock is increased. However, it is impossible to reduce the resolution due to continuous dimming characteristics. Also, it is not preferable to increase the timer clock in consideration of power consumption or heat generation.